We’ve all heard that old saying, “A fish out of water”. Unusually meant to describe something that makes you feel uncomfortable, completely out of your environment, etc. Though it was previously believed that fish had evolved to survive, at least briefly, on land once or twice it is know believed, thanks to a new study, that they can survive dozens of times.
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Though the transition between water and land is extreme, it is not necessarily difficult. Well, what does this mean?

Well, it means something else prevent fish from becoming full-time, land-swelling creatures. The authors of the study, published in Evolution in August, ask a fundamental question: “How do species establish themselves in new environments?”

It can be gathered that most species fail to establish themselves in new environments…most of the time. For fish, there are many hurdles to overcome such as breathing, moving, and metabolizing. All of these things, and even more bodily functions, have to be modified in a way that accepts radically increased gravity and seriously decreased wetness.

Authors of the study, Terry Ord and Georgina Cooke of the University of New South Wales, had a few theories, as summarized below.

Fish that live in the intertidal zone would be more likely to have contact with land and away from their water due to their twice-daily transformation of the tides. Similarly, fish that live in ponds, puddles, or creeks that shrink and grow face a similar challenge.

Fish that live in water that is prone to heating up are known to lave it because warmer water means less oxygen, which leads to suffocating. In these cases, air/land provide some relief.

Fish that live on the bottom of their body of water also seem like better candidates for land-dwelling due to the fact that they have certain adaptations that make them more suited to the ground, such as flattened bodies that make walking easier and fins that are limb-like.

Since getting around on land is difficult for any fish, it might be easier for those whose diets don’t depend on seeking out mobile prey.

Based on these, they began a deep dive of all fish species that fit the above parameters. They found amphibious behavior in about 130 fish, from 33 different families that reach the oldest and youngest branches of the fish family tree. Many of these 33 families have a great evolutionary chance to transition from water to land.

But let’s get back to the question at hand – why have so few fish pioneered into the terrestrial world, because breathing, walking, and reproducing do not seem to pose huge barriers.

The authors theorize that, at the root of the issue, is staying wet. Fish drying out is the real challenge, especially the desiccation gills. Gills must remain moist in order for a fish to breathe, and without that guaranteed moisture…they’re doomed.